Facsimile machines are made which include modems having automatic gain control (AGC) circuitry. Such facsimile mach1nes are used for the transmission of drawings or writings from point-to-point using a communications system, such as a telephone network. The transmitting facsimile machine operates mechanisms for scanning parallel lines across the drawing or writing in order to detect markings, and by sending electrical data signals during each line of the scanning, with the data signals corresponding to the intensity of the markings. The receiving facsimile machine operates mechanisms for scanning parallel lines across a blank sheet of paper for printing markings corresponding to the electrical data signals received from the transmitter facsimile machine, thus reproducing the markings detected by the transmitting facsimile machine. One particular protocol for the data signals, known as CCITT Group II format, calls for the production of signals having high energy, "white" portion for the initial portion of eacn scanning line, and calls for data signals having a magnitude which increases with increases in the intensity of markings.
A particular problem faced by Group II modem receivers is that the time averaged energy of incoming received data signals is not constant, but varies with the intensity of the markings being scanned. That is, the energy output of a Group II transmitter is a function of the base-band modulating input, with black and white markings causing minimum and maximum transmitted energy, respectively, with shades of grey falling somewhere between these extremes. Facsimile machine protocols other than Group II may include scrambling algorithms in which uniform average energy communications are produced. Conventional AGC loop circuitry senses the incoming data signal energy and adjusts a gain control accordingly in order to produce the level of amplification necessary. The use of such conventional AGC circuitry with Group II protocol is not possible because the transmitted energy is not uniform but varies with the intensity of the markings being scanned.
An AGC technique which has been used with prior Group II facsimile modems requires the use of circuitry to synchronize the modem with the start of each line, and to use a strobe pulse generated at the start of each line to enable the AGC gain control so that AGC adjustments are made only during the initial portions of lines, when the incoming data signal has maximum energy. The strobe pulse is typically generated by the facsimile mechanism controlling circuitry and is provided to the modem circuitry. Such a strobe technique is inconvenient, particularly when a standardized modem AGC circuit is to be constructed for use by a large number of different facsimile machine manufacturers having differing facsimile machine constructions which have differing strobe pulse characteristics.
The Group II facsimile protocol transmissions proceed, in sequence, by sending a framing header, waiting for acknowledgment signals from the receiving facsimile machine, sending the page of the drawing or writing, ceasing transmissions, and resuming the transmission sequence with a new framing header and repeating the sequence until all pages have been sent. The framing header consists of thirty-six lines sent serially, with each line consisting of an initial "black" (low energy) portion having a duration equal to 4% to 6% of a line duration followed by the remainder of the line which is totally "white" (high energy). The purpose of the framing header is to allow the mechanisms of the receiving facsimile machine printer to become synchronized with the mechanisms of the transmitting facsimile machine scanner. Conventional Group II modem AGC circuits, as described above, rely on the facsimile machine synchronization to control the strobing of AGC update adjustments.
After the framing header has been received, the receiving facsimile machine answers back with signals acknowledging receipt of the framing header and indicating readiness to receive the page data. The page data consists of a large number of lines sent serially, with each line consisting of an initial "white" (high energy) portion having a duration equal to 4% to 6% of a line duration followed by the remainder of the line which has an energy level which varies with the intensity ("black" to "white") of the markings being scanned.
The Group II protocol uses a vestigial sideband, suppressed carrier form of amplitude modulation in which predominantly "white" markings are associated with large signal amplitudes and predominantly "black" markings are associated with small signal amplitudes. The Group II protocol provides that facsimile data is to be communicated digitally, as binary ones and zeros, in a serial bit stream. When a Group II binary one is transmitted, the instantaneous amplitude and energy transmitted becomes very large. Therefore, when a "white" marking is scanned, mostly binary ones are transmitted and tne amplitude and energy transmitted remains large during "white" transmissions. When a Group II binary zero is transmitted, the instantaneous amplitude and energy transmitted becomes very small. Therefore, when a "black" marking is scanned, mostly binary zeros are transmitted and the amplitude and energy transmitted remains small during "black" transmissions.